Transmissions typically include an input shaft, an output shaft, and a collection of interrelated gear elements, such as in a planetary arrangement or otherwise. These elements are used to selectively couple the input shaft and the output shaft. Hydraulic clutches are also well known, and can be found in many systems and devices. In one implementation, a set or a plurality of hydraulic clutches facilitates shifting of a transmission between differing input/output gear ratios or ratio ranges. The clutches are used to select gear ratios in a discrete transmission, and to select gear ratio ranges in a continuous transmission. Both types of coupling are referred to herein as “ratios.”
Selection of a gear ratio at the output shaft is executed through one or more clutches that affect the rotations and/or interrelationships of the gear elements. The clutches are typically hydraulically actuated to engage band or disk torque transfer elements. Shifting from one gear ratio to another typically involves releasing or disengaging an off-going clutch or clutches associated with a current gear ratio and applying or engaging an on-coming clutch or clutches associated with the desired gear ratio. Although different clutch arrangements may be provided within such transmissions, one arrangement is a “two-clutch” shifting transmission. In this arrangement, the two clutches for each gear include a primary clutch, often a rotating clutch element, which is retained for an upcoming gear, and a secondary clutch that is disengaged in order to shift into the upcoming gear. The secondary clutch for this shift condition is often referred to as the off-going clutch. This clutch is replaced by a new clutch, namely the “on-coming” clutch, that actuates the transmission into the new gear. In other words, a shift is executed by deactivating a single “off-going” clutch, activating a single “on-coming” clutch, and in some cases holding a third clutch for both the old and new gears. In other arrangements, multiple on-coming and\or off-going clutches are employed, increasing the complexity and criticality of clutch actuation timing.
Each hydraulic clutch in these arrangements is typically driven through an electrically controlled solenoid valve. Each of the solenoid valves is modulated through the application of an electrical control signal to control hydraulic fluid pressure applied to the clutch and, as a result, to control a clutch piston movement during various phases corresponding to a shift.
The manner in which phasing of the on-coming and off-going clutch element occurs can have a substantial impact on the perceived shift quality. For example, if the off-going clutch disengages prematurely, the engine speed may surge briefly before the on-coming clutch, if still in a fill phase, possesses sufficient torque capacity. If the on-coming clutch fills prematurely, the clutch element has significant torque capacity prematurely, i.e., before the off-going clutch is ready to commence torque transfer. This often results in a three-way clutch tie up which is detrimental to the transmission's useful life in a mild case, and which can result in mechanical damage to the transmission in a more severe case.
On the other hand, in the event of a late clutch fill, the off-going clutch attempts to hand off torque to the on-coming clutch before the on-coming clutch has sufficient torque capacity. As a result, the transmission slips as the on-coming clutch fails to lock with adequate torque capacity to hold the specific gear in question. The end result is a slip phenomenon in the clutch discs, also an undesirable event as this tends to produce high clutch energies resulting from excessive heat generation produced by the higher clutch relative velocities of the rotating clutch discs.
In addition to creating an unpleasant user experience, improperly timed shifting will adversely impact the efficiency and service life of the transmission. To this end, it is desirable to actuate the clutches with precision such that a smooth shift occurs throughout the entire operating speed range of the transmission during its entire useful life.
Known methods for providing a control strategy to exchange or “hand-off” clutches in an electronic control power transmission include a dual control, time-based strategy. That is, the clutch fill time in which the on-coming clutch transitions from a hold phase to a modulation phase is matched with the off-going clutch as a function of the timing of various clutch events. This control strategy is based upon the determination of the on-coming pressure at release, the clutch unlock pressure and an on-coming clutch pressure past release. While this control strategy generally produces acceptable shift quality, it is often somewhat difficult to tune due to the many variables that must be simultaneously considered and balanced in performing the clutch hand-off. When such parameters are not adequately addressed, the transition may result in a degradation of shift quality.